Generation of electrical power is increasingly relying on sources of renewable energy, including photovoltaic (PV) cells or solar cells. However, the cost of solar cells remains high and has heretofore prevented the wide commercial use of solar cells in consumer applications. Many research efforts focus on reducing the processing and materials cost or increasing the efficiency of the PV conversion from solar radiation to electrical energy. As a result, although PV devices are still mostly based on silicon, a multitude of other materials and device architectures have been investigated, including multiple junctions of III-V semiconductors, cadmium telluride (CdTe), copper indium gallium selenide (CIGS) combinations, organic films, and, more recently, perovskites and two-dimensional (2D, monolayer) atomic crystals. Among the latter, transition metal dichalcogenides (TMDCs or TMDs) have some unique properties and technological advantages which make them ideal candidate for high-efficiency, low-cost and stable PV devices, both in rigid and flexible form. Few to single atomic layers of TMDC are direct semiconductors with band-gap between 1 and 3 electron volts (eV). A large absorption per thickness has been reported for several TMDCs, including molybdenum disulfide (MoS2) and molybdenum diselenide (MoSe2). Absorption in a wide range of wavelengths can be achieved in a single TMDC by elastically straining it to a great extent. TMDCs can be stacked onto many different substrates, thereby offering an opportunity to create broad band absorbers to better match the solar spectrum. Recent progress into effectively printing TMDCs suggests an economical approach to their deposition over a large area.
A variety of heterostructure solar cells based on TMDC has been demonstrated, including MoS2/WSe2, MoSe2/WSe2, MoS2/Graphene, MoS2/InP, α-MoTe2/MoS2, and MoS2/p-Si. However, the reported absorption and conversion efficiencies are low, in the order of 5-10% and a few percent, respectively. These poor performances are due, at least in part, to a limited absorption of the solar radiation in active layers with atomic scale thickness. Unfortunately, TMDCs with a thickness larger than three atomic layers are indirect semiconductors with little to no absorption of the solar radiation.
Therefore, in order to harness the potential of TMDCs in photovoltaic applications, a new solar cell architecture is needed to enhance collection of light.